When a press work is performed, for example, on a work object (e.g. a long metal plate) that has been wound in a coil (or roll) configuration, it is necessary to feed the work object to a press work apparatus at a predetermined (or desired) feed speed (or ratio) while leveling it to eliminate winding strain. To this end, various types of so-called straightener feeders have been developed.
When positioning of the work object is to be performed in a press work apparatus, it is required for the straightener feeder to release clamping of the work object to set it free with respect to the plane of the work object or at least with respect to the work feeding direction, in order to attain precise positioning and to prevent deformation of the work object. Similarly, at the time when working that involves deformation of a work object is performed, it is required for the straightener feeder to release the work object from the clamped state.
A straightener portion and a feeder portion of a straightener feeder generally have a roll(s) to be in contact with the front surface of the work object and a roll(s) to be in contact with the back surface of the work object, and the work object is pressed between these rolls. It is required for the straightener feeder to be able to perform an opening operation, that is, an operation in which the roll(s) disposed on the front side of the work object and the roll(s) disposed on the back side of the work object are spaced apart to a relatively large extent at the time when the leading edge of the work object is to be inserted into the straightener portion of the straightener feeder upon threading or when maintenance such as cleaning of the rolls or other portions is to be performed as occasion demands.
Japanese Patent Application Laid-Open No. H10-94830 and Japanese Utility Model Application Laid-Open No. H05-70719, describe a frame that supports rolls disposed on the front side of the work object are adapted to be able to move upwardly utilizing back and forth movement of an actuator such as a cylinder thereby moving the rolls disposed on the front side of the work object away from rolls disposed on the back side to perform the release operation and the opening operation.
According to a technology described in Japanese Utility Model Application Laid-Open No. H05-88706, an eccentric shaft is turned by an electric motor, and a frame that supports rolls disposed on the front side of the work object is adapted to be able to move upwardly utilizing the eccentricity of the eccentric shaft thereby moving the rolls on the front side of the work object away from rolls on the back side to perform the release operation.
In the apparatuses described in Japanese Patent Application Laid-Open No. H10-94830 and Japanese Utility Model Application Laid-Open No. H05-70719 mentioned above, the release operation is performed using a cylinder in synchronization with press work that is performed at a relatively short cycle time, and accordingly a large noise is generated upon switching of an electromagnetic valve and/or upon collision occurring at cylinder ends. Therefore, there is a demand for reducing such noise to improve the working environment.
In the case where a cylinder is used, there is a relatively long delay time since a clamp signal for causing the straightener feeder to terminate the releasing state and clamp the work is generated until the operation pressure actually reaches a predetermined level. For this reason, it is not possible to adapt the release operation in such a way as to satisfactorily meet demands for increases in the number of strokes of press work per unit time (or increases in press work speed).
In the arrangement in which the released state is terminated by rotating an eccentric shaft using an electric motor as disclosed in Japanese Utility Model Application Laid-Open No. H05-88706, the amount of eccentricity in the eccentric shaft is determined in advance in adaptation to the release operation, and if the amount of eccentricity is to be changed, it is needed to replace the eccentric shaft with another shaft having a different amount of eccentricity. Accordingly, it is difficult to open the rolls to a large extent. Therefore, it is necessary to provide a separate mechanism for performing the opening operation to allow to perform, for example, cleaning of the rolls and threading of a work object into the straightener feeder, in addition to the release mechanism operated by rotating the eccentric shaft with the electric motor. This disadvantageously leads to an increase in the size and complexity of the apparatus and causes various problems in terms of cost and ease of installation, assembly and maintenance of the apparatus.